2019 UMN Cooperative Black Cutworm Trapping Network Report #7

Report #7 Week of May 18-24, 2019

For more information: Black Cutworm Reporting Network

May 30, 2019

Black cutworm (BCW) moth activity was well down this week. The exception was a May 24-27 flight into southeast MN that was picked up by traps in Goodhue, Dodge, and Freeborn Counties (Figure 1).  


Figure 1. Maximum two consecutive night black cutworm captures reported by county.

While this was the last week of full scale trapping, additional moths will likely continue to move into the state over the next few weeks. However, some of these moths can be shorter distance migrants and much of southern MN has already seen earlier flights whose offspring will be active for quite some time. A few traps, including Lamberton will continue operating for a while over curiosity or concerns for late-planted sweet corn and other crops.

There was at least one typo in last week’s newsletter. I mentioned the possibility of black cutworm north of Stevens and Polk Counties. This should have read Stevens and Pope Counties. Well, just to bail me out, the Polk County trap did pick up BCW over the weekend so the moths are making their annual journey to northern parts of the range.

Table 1 shows degree-days for BCW larval development and risk for corn damage for the significant captures so far this spring… and there are a lot of them (Figure 2). Some of these traps have captured quite a few moths this season (Figure 3)!

Earlier planted corn is emerging. Based on the earliest flights, scout corn before 300 degree-days after moth arrival or June 1.  Remember, it takes larvae 4th instar (stage) or larger to cut corn. Also, corn cut above the growing point will survive. However, broadleaf crops are easily killed by the feeding of black cutworm (and other cutworm species) when they are cut below the cotyledons. Small seedlings, sugarbeets for example, can be cut by cutworms smaller than 4th instar.


Table 1. Estimated black cutworm development for significant moth captures as of May 28. Current and forecast degree-days are generated from the U2U Corn Growing Degree-Day application at the Midwest Regional Climate Center https://mrcc.illinois.edu/U2U/gdd/ using current year and NWS forecast temperatures.

Economic thresholds - When to treat a problem 

stand and yield loss table

Table 2. Corn yield response to plant population (Morris, Lamberton and Waseca (2009-2011) Source: Dr. Jeff Coulter, UMN

Cutworms reduce yield by decreasing final stand or plant population. The generic economic threshold for black cutworm in corn is 2-3% of the plants cut or wilted when the larvae are less than ¾ inch long. The threshold increases to 5% cut plants when larvae are larger. However, with high corn prices, these thresholds could be lowered to 1% wilted or cut and small larvae and 2-3% wilted or cut for large larvae.

Remember to take into consideration corn populations in individual fields and adjust threshold numbers accordingly. For example, if the current plant population is at or near yield limiting levels, you can afford to lose fewer plants than in a field with a higher emerged population. The role of corn plant stand in determining yields can be found in Table 2. 

The reason the black cutworm economic threshold varies by larval size is based in larval feeding. Cutworms must shed their skins (molt) in order to grow. The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~1/2 inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are full grown and cease feeding between 1½ and 2 inches long. While larger larvae will cut or tunnel into larger plants, they have less time left to feed and, as a result, have the potential to cut fewer plants. Table 3 gives approximate sizes in length and width of the head for black cutworm larvae.

BCW growth characteristics table

Table 3. Black cutworm body and head capsule sizes by stage.

There are more detailed dynamic black cutworm thresholds available. They use stand, crop stage, projected damage and crop price. However, caution is advised when dynamic thresholds generate lower thresholds below those described above. Yield loss, actual or measurable, does not begin with the first missing corn plant. If there were high grain prices and a good planted and emerged stand, you could easily be treating cutworm populations that would not reduce stand enough to actually hurt yields.

The rescue insecticide calculator (Table 4) is adapted from a University of Illinois publication and is an example of a dynamic threshold that is used in several management guides. Modern corn yields and prices could indicate treatment at a very low percentage cut plants using this worksheet, perhaps leading to over-reactive treatment decisions. However, the yield loss factors are still useful when combined with yield loss by stand reduction charts.


Table 4. Yield loss factors and equations to calculate the profitability of a rescue insecticide treatment for black cutworm. Source: http://extension.cropsciences.illinois.edu/fieldcrops/insects/black_cutw...


Figure 2. Weekly captures by county.


Figure 3. Season total captures by trap (Locations may be accurate for ease of readability).

Thanks to all the Cooperators who check and reported pheromone trap captures!  There are likely interns and others who helped as well. Thanks to all.

 

This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.

Until next week,

Bruce Potter and Travis Vollmer